This invention relates to methods and materials for thermal imaging. More specifically, this invention relates to certain mixed carbonate esters of quinophthalone dyes, to the use of these mixed esters in thermal imaging, and to heat-sensitive elements containing these mixed esters.
A variety of thermal imaging systems for producing color images have been proposed, and several have been mentioned in Kosar, J., Light-Sensitive Systems: Chemistry and Application of Nonsilver Halide Photographic Processes, New York, John Wiley and Sons, Inc., 1965, pp. 402-19. In one type of heat-sensitive recording system, a first sheet containing a first reagent is superposed over a second sheet containing a second reagent and one of the reagents is melted or vaporized by the imagewise application of heat and transferred for reaction with the other reagent to form a color image. In another type of dye transfer system, images are formed by sequentially transferring two or more dyes carried on separate donor sheets to a common receptor sheet by melting or volatilization. In thermal imaging systems of the "self-containing" type, a single sheet is used and the imagewise heating of the heat-sensitive sheet produces a color image, for example, by rendering a coating layer transparent to reveal the color of a background layer, by initiating the chemical reaction of two or more reagents to form a colored product or by bleaching, coloring or changing the color of a single reagent. In most of the non-silver thermal imaging systems in commercial use, color images are formed using two or more reagents that usually are encapsulated or otherwise isolated from each other until melted and mixed upon imagewise heating.
A number of compounds which undergo a color change from a colorless to a colored form, from one color to another color or from a colored to a colorless form upon application of heat have been disclosed. For example, U.S. Pat. No. 3,723,121, issued Mar. 27, 1973, discloses several thermochromic materials for laser beam recording including inorganic compounds, such as black copper (II) oxide which decomposes to red copper (I) oxide upon heating, and organic compounds, such as polyacetylene compounds which subsequent to treatment with ultraviolet light undergo two changes in color, first to red then to yellow, as the temperature is increased. U.S. Pat. No. 4,242,440, issued Dec. 30, 1980, discloses another class of heat-sensitive polyacetylene compounds which exhibit color changes, for example, gold to red, brown to orange and gold to orange, which color changes are reversible. U.S. Pat. No. 3,488,705, issued Jun. 1, 1970, discloses thermally unstable organic acid salts of triarylmethane dyes useful in electrophotographic elements as sensitizing dyes that are decomposed and bleached upon heating. U.S. Pat. No. 3,745,009, originally issued Jul. 10, 1974 and reissued as Reissue No. 29,168, and U.S. Pat. No. 3,832, 212, issued Aug. 27, 1974, disclose heat-sensitive compounds for thermography containing a heterocyclic nitrogen atom substituted with an --OR group, for example, a carbonate group, that decolorize by undergoing homolytic or heterolytic cleavage of the nitrogen-oxygen bond upon heating to produce an RO+ ion or RO- radical and a dye base or dye radical, which may in part fragment further. U.S. Pat. No. 4,380,629, issued Apr. 19, 1983, discloses styryl-like compounds which undergo coloration or bleaching, reversibly or irreversibly via ring-opening and ring-closing in response to activating energies such as light, heat, electric potential and so on.
U.S. Pat. No. 4,720,449, issued Jan. 19, 1988, describes a thermal imaging method which comprises heating imagewise a di- or triarylmethane compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom directly through a nitrogen atom, which nitrogen atom is also bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of the nitrogen atom to form a new group in the ortho position, whereby the di- or triarylmethane compound is rendered colored in an imagewise pattern corresponding to the imagewise heating.
U.S. Pat. No. 4,663,518, issued May 5, 1987, describes an identification card comprising a single medium having a plurality of different heat sensitive image forming dye compounds on which are encoded a colored pictorial image of the card holder, colored text, and machine readable digital data. A laser printing method is provided for activating the heat sensitive image forming dyes to provide the colored pictorial image of the card holder, colored text, and machine readable digital data.
U.S. Pat. No. 4,602,263, issued Jul. 22, 1986, and U.S. Pat. No. 4,826,976, issued May 2, 1989, both describe thermal imaging systems for optical recording and particularly for forming color images. This thermal imaging method relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift from colorless to colored, from colored to colorless or from one color to another. These patents also describe an imaging dye of the formula: EQU [M(X).sub.q ].sub.p D
wherein M is a carbamate moiety: X is --N.dbd., --SO.sub.2 --, or --CH.sub.2 --; D taken with X and M represents the residue of an organic dye; q is 0 or 1; and p is a whole number of at least 1 and usually is a whole number of 1 to 3. Preferably, M has the formula: ##STR2## wherein R is alkyl usually having 1 to 4 carbon atoms; --SO.sub.2 R.sub.1 wherein R.sub.1 is alkyl usually having 1 to 6 carbon atoms; phenyl; naphthyl; or phenyl substituted with alkyl usually having 1 to 6 carbon atoms, alkoxy usually having 1 to 6 carbon atoms, halo, such as chloro or bromo, trihalomethyl, such as trichloromethyl or trifluoromethyl, cyano, nitro, carboxy, --CONR.sub.2 R.sub.3 wherein R.sub.2 and R.sub.3 each are hydrogen or alkyl usually having 1 to 6 carbon atoms, --CO.sub.2 R.sub.4 wherein R.sub.4 is alkyl usually having 1 to 6 carbon atoms or phenyl, --COR.sub.5 wherein P.sub.5 is amino, alkyl usually having 1 to 6 carbon atoms or phenyl, --NR.sub.6 R.sub.7 wherein R.sub.6 and R.sub.7 each are hydrogen or alkyl usually having 1 to 6 carbon atoms, --SO.sub.2 --NR.sub.8 R.sub.9 wherein R.sub.8 and R.sub.9 each are hydrogen, alkyl usually having 1 to 6 carbon atoms or benzyl; and Z is an acyloxy group, --COOR.sub.10, wherein R.sub.10 is a tert-alkyl or --(CH.sub.2).sub.2 Y group, wherein Y is an electron-withdrawing group. It is stated in column 4 of each patent that D may be the residue of:
a triarylmethane dye, a xanthene dye, a rhodamine dye, a fluoran dye, an azocarbocyanine dye, a benzylidine dye, a thiazine dye, an acridine dye, an aminoanthraquinone dye or other dye containing a nitrogen atom possessing a lone pair of electrons, which when substituted with said acyloxy group to form said carbamate moiety, will be color-shifted. Preferably a tert-alkoxy carbonyl group, such as, tert-butoxycarbonyl is used as the acyloxy group to isolate the lone pair of electrons on the nitrogen atom of the organic dye to effect said color shifting. The fragmentation of the resulting carbamate moiety upon application of heat releases said lone pair of electrons to effect a visually discernible change in the spectral absorption characteristics of the dye, preferably, from colored to colorless or colorless to colored.
The use of tert-butoxycarbonyl (hereinafter "t-BOC") as a protecting group for nitrogen functionalities is well known, and the thermal lability, i.e., instability of this protecting group at 150.degree.-170.degree. C. is mentioned in Greene, Theodora W., Protective Groups in Organic Synthesis, New York, John Wiley and Sons, Inc., 1981, page 326.
Despite the development of all the prior art dyes discussed above, there are still difficulties in producing a satisfactory yellow image in a multi-color thermal imaging system. Many proposed dyes undergo a change from yellow to colorless on exposure to heat, whereas for imaging purposes it is preferred that the color change be from colorless to colored. Many yellow thermal imaging leuco dyes (the term "leuco dye" is used herein to refer to a substantially colorless compound which generates a colored material upon heating) undergo slow thermal breakdown at room temperature, thus restricting the shelf life of imaging materials containing the leuco dye, especially in warm climates. Furthermore, it is difficult to produce a yellow thermal imaging dye in which the absorption shift between the leuco dye and the dye is so large that the leuco dye is essentially colorless, while the yellow dye provides high extinction in the yellow region, and is sufficiently stable that the resultant image does not fade upon extended storage; many proposed yellow leuco dyes have a pale yellow color, which undesirably affects the background color of any image produced.
Accordingly, there is still a need for a yellow thermal imaging leuco dye which is stable for long periods at room temperature, which is substantially colorless so that it does not affect the background color of an image. This invention provides leuco dyes, and methods and materials for thermal imaging which meet these requirements and which produce high quality yellow images. Related compounds of the invention also make good orange to red dyes which may be used to produce high quality images.